Bathymay : la structure sous-marine de Mayotte révélée par l'imagerie multifaisceaux

International audienceLa campagne Bathymay, effectuée en 2004 à bord du N/O Marion Dufresne 2 , a permis de réaliser le premier levé bathymétrique complet de la pente insulaire de Mayotte. L'analyse préliminaire de la morphologie révèle l'existence de nombreux canyons, de vastes déstabilisations de flancs et d'édifices volcaniques sous-marins. Des falaises abruptes, dont certaines semblent guidées par des failles, limitent de vastes plateaux sous-marins. La campagne Bathymay a mis en évidence deux mécanismes de déstabilisation du volcan de Mayotte : la déstabilisation lente, issue de l'effondrement gravitaire de l'édifice pendant le stade bouclier, puis la déstabilisation catastrophique des flancs du volcan au stade post-bouclier The BATHYMAY marine survey was carried out in January 2004 around the French volcanic island of Mayotte, onboard the research vessel Marion Dufresne 2. Multibeam bathymetry revealed for the first time the morphology of the whole outer slope. Preliminary interpretations show broad canyons that deeply incise the slope, numerous volcanoes and huge landslides. Large submarine plateaus are also highlighted, bounded by steep cliffs and active normal faults. A model for Mayotte is suggested, in which eruptive activity, sometimes influenced by normal faulting, was concentrated along a N140◦ regional line (the Comoros archipelago axis), with landslide activity concentrated on the northeastern and southwestern flanks of this line

Offshore Oligo-Miocene volcanic fields within the Corsica-Liguria Basin: Magmatic diversity and slab evolution in the western Mediterranean Sea

International audienceThe European and Corsica-Sardinia margins of the Ligurian Sea (western Mediterranean) have been affected by a geochemically diverse igneous activity, offshore and onshore, since the Eocene. This magmatism occurred in a global subduction-related framework. On the European side, the oldest Tertiary magmatism dated at ca. 35 Ma was mainly calc-alkaline. It included the emplacement of plutonic bodies of adakitic affinity, such as the quartz microdiorite laccolith locally referred to as "esterellite". Younger magmatic events on-land within the whole Ligurian domain were mostly medium-K or K-rich calc-alkaline. Miocene volcanic activity was important in Sardinia, where andesites and ignimbrites were erupted during several magmatic cycles. In Corsica, it was minor although it emplaced lamprophyres near Sisco at 15 Ma. Dredging and diving cruises conducted in the Ligurian Sea during the last thirty years allowed us to collect a number of submarine samples. We discuss here their geochemistry (major and trace elements) and their whole-rock K-Ar ages and mineral 40Ar-39Ar plateau ages. Around 15 Ma, minor amounts of adakitic lavas were emplaced off southwestern Corsica, in the deepest part of the Liguria-Corsica Basin. They rested over the thinnest southwestern Corsica Hercynian continental crust. Closer to the coast, contemporaneous calc-alkaline rocks erupted on a less thinned crust. The adakitic events could be indicative of either the final stages of active subduction, or alternatively of a slab tearing linked to the southeastern retreat and steepening of the slab. The latter event could be connected with the end of the Corsica-Sardinia block drifting and its correlative eastern collision. Younger volcanic effusions, dated at 14-6 Ma, occurred mostly northwest and north of Corsica. K-rich calc-alkaline basalts, shoshonites and K-rich trachytes were emplaced during this period, and alkali basalts erupted as early as 12 Ma in Sardinia. In the Toulon area, alkali basalts dated at 7-6 Ma represent the last onshore activity just before the Messinian crisis, and the Pliocene alkali basaltic outpouring in Sardinia. We propose to link these latter volcanic events to the development of a slab window in a post-collisional tectonic framework

Cenozoic tectonics of the Western Approaches Channel basins and its control of local drainage systems

International audienceLe domaine des Approches occidentales de la Manche constitue une zone clé pour caractériser l'évolution post-rift des marges continentales NW européennes associées à la collision Afrique/Europe. Malgré les divers témoins des inversions cénozoïques jalonnant le pourtour de la Manche, la structuration et l'amplitude des mouvements demeurent néanmoins incertaines au sein de la partie méridionale française des Approches occidentales. Il en est de même sur le rôle de l'inversion de la mer du Nord dans la mise en place du Fleuve Manche qui drainait un bassin versant bien supérieur à la Manche actuelle durant les grandes régressions plio-quaternaires et alimentait les éventails sous-marins Celtique et Armoricain en bas de pente. La réalisation des campagnes de sismique-réflexion haute résolution GEOMOC et GEOBREST03 dont les résultats font l'objet de cet article permettent de répondre à ces questions en complétant la connaissance géologique de la Manche. Les nouvelles observations soulignent le diachronisme et le contraste de l'amplitude des mouvements du système de failles associées à l'inversion du bassin d'Iroise. Celle-ci se fait en deux épisodes: un épisode paroxysmal paléogène décomposé en deux phases, éocène (Yprésien probable) et oligocène, et un épisode néogène plus modéré réactivant partiellement les structures impliquées antérieurement. Les déformations se concentrent le long de l'accident nord Iroise (NIF) situé dans le prolongement de la faille Médio-Manche et entraîne localement des plissements de la couverture sédimentaire à l'aplomb des accidents profonds. L'inversion induit ainsi un soulèvement de près de 700 m du plateau médian situé au sud de l'accident nord Iroise. La cartographie isochrone des séquences sismiques identifiées démontre également le contrôle majeur des structures tectoniques sur la mise en place des dépôts néogènes. Le soulèvement de la partie orientale du bassin favorise ainsi la mise en place de vastes prismes progradants d'âge miocène supérieur, et contrôle le développement postérieur du réseau des paléo-vallées constituant l'extrémité occidentale du fleuve Manche. Ce réseau présente une géométrie en baïonnette marquée par de brutaux changements de directions variant de N040 à N070, cette dernière direction caractérisant la plus grande partie des failles néogènes associées au bassin d'Iroise. Les paléo-vallées se seraient développées lors d'une chute du niveau marin au-delà du rebord de plate-forme et la stratigraphie établie à travers cette étude amène à placer le début des incisions au Pliocène (Reurévien ou pré-Tiglien). La chute amplifiée par l'inversion du bassin d'Iroise serait suivie d'un basculement tardif de la plate-forme externe à l'instar des observations réalisées sur de nombreuses marges du pourtour nord atlantique

East-Corsica basin - Regional setting, MSC Surfaces, MSC Basinal Units.

International audienc

Campagne océanographique GEOBLAVET

Campagne en mer réalisée du 15/09/2001 au 28/09/2001 par le LABORATOIRE GÉOSCIENCES OCÉAN (LGO) - UMR 6538 sur le ThaliaStudy of the Blavet (south Brittany) paléovalley system and of the different recent sedimentary units in the coastal areas of Southern Brittany in relation with the results of GEOVIL (June 1998) and GEODET (June 2000) surveys. In September 2000, this was to contribute to the specification, comparison and quantification of the various deposit sequences preserved on the scale of the contemporary South Brittany domain, in the recent history of the Armorican Massif. The cruise fell under the COTARMOR project (headed by Jean-Noel Proust).Exploration de la nappe sédimentaire côtière entre la cote et l'isobathe 50 mètres en Bretagne Sud. Cette étude porte principalement aux débouches: (1) du Blavet et du Scorff, et de son raccordement avec l'Odet (2) de la rivière d'Etel (3) du Mors Bras (Baie de Quiberon et de Vilaine). Cette campagne complète et assure le lien avec les 2 précédentes missions GEOVIL (Juin 98) et GEODET (Juin 2000). Septembre 2000. Il sera alors possible de préciser, comparer, quantifier les différentes séquences de dépôt préservées à l'échelle du domaine sud breton contemporain de l'histoire récente du Massif armoricain. Le projet de rattachement est COTARMOR (Responsable Jean-Noel Proust)

West-Corsica Margin - Regional setting, MSC Surfaces, MSC Basinal Units.

International audienc

Refraction/wide-angle reflection investigation of the Cadomian crust between northern Brittany and the Channel Islands

International audienceA seismic refraction experiment has been carried out off the coast of northern Brittany (NW France) to investigate the deep structure of the North Armorican block. The seismic source was an array of airguns and the shots were recorded simultaneously both by ocean bottom seismometers and land stations. We present 2D velocity models computed from two crossing seismic refraction profiles (labelled P1 and P2 and each about 135 km long) oriented NE-SW and NNW-SSE, i.e. parallel and perpendicular, respectively, to the regional basement structures. Wide-angle reflection profiles were also performed to extend the 2D structures to a 3D crustal interpretation. Observed and calculated P-wave traveltimes were matched by forward modelling prior to their inversion in order to produce two crustal velocity models. The model which best fit the data consists of: (1) high basement velocities at the top of the crust, except in an area with shallow seismic velocity which can be as low as 3.0 km/s at the surface and possibly related to the existence of shallow Paleozoic sedimentary basins; (2) the mean crustal velocity ranges from 5.5 to 6.8 km/s at a depth of 15 km depth; (3) lower crustal velocities range from 7.0 to 7.5 km/s at a depth of 35-38 km and then from 7.5 to 8.1 km/s at the crust-mantle boundary located between 35 and 38 km depth. Comparisons with the crustal section imaged by the coincident seismic vertical reflection profile SWAT 10 show a good correlation between velocity gradients and lower crustal reflectivity. When compared to surrounding basement areas, the North Armorican Cadomian crust displays a slightly thicker crust and also a thicker reflective lower crust which can be related to the relative preservation of this Upper Proterozoic block from later Paleozoic and Mesozoic tectonic episodes. The crustal structure of the North Armorican block thus appears to be an intercalated feature between the surrounding Paleozoic areas and older shield or platform domains

Sedimentary architecture of the Loire River drowned valleys of the French Atlantic shelf

International audienceThe analysis of a dense grid of high resolution seismic profiles collected offshore the present day Loire River estuary indicates the presence of a thick and complex Pleistocene coastal wedge between the coast and 50 meters water depth. Most of this coastal wedge is preserved in a fossil valley network starting 10 km off the coast on the “Precontinent Breton” topographies and wedging out progressively 50 km in the offshore where the shelf flattens. This system is comprised of three main valleys 30 km long, 40-60 m deep and 0.7 to 4 km wide each, in average, with a northern valley incompletely filled by sediment. These valleys are incised into Eocene (Ypresian-Bartonian) sedimentary rocks lying unconformably on the metamorphic and magmatic rocks of the South Armorican Massif basement. The coastal wedge is comprised of six seismic units. From the base of the valleys to the seafloor, these units are successively interpreted as (1) colluvial (U1) and braided river deposits (U2), overlain by restricted marine to estuarine sediments (U3), and (2) straight to meandering fluvial deposits (U4) giving rise vertically to floodout marine sediments (U5). The whole sediment pile is capped by open marine bioturbated mudstones (U6). This succession of seismic units is organised in two depositional sequences bounded by an unconformity of regional extent, which corresponds to a drastic change in the paleovalley fill architecture. The lower sequence fills up the southern and central valleys when the upper sequence fills up the northern valley network. Both sequences are sharply truncated by a ravinement surface at the base of the offshore shales formed during the Holocene marine transgression. The correlation of the observed depth of the incisions and transgression surfaces with the global sea-level curve provides an indirect estimate of the ages of the depositional sequences. The lower sequence is probably Saalian (130-200 Ka, MIS 6) and the upper one Weischelian (110-12 Ka, MIS 2-4 and 5a-d). The ages and the infill of these valleys are very close to the ones observed in the largest paleovalleys of the inner French Atlantic shelf (Gironde, English Channel) and consistent with the sedimentary record of the continental Loire River (stepped terraces). In the later case, the Saalian period corresponds to a sharp increase on the incision of the river that shaped the morphology and determined the location of the present-day Loire valley